(a) Field of the Invention
The present invention relates to an undrawn polyethylene terephthalate (PET) fiber, a drawn PET fiber, and a tire cord including the same. More particularly, the present invention relates to an undrawn PET fiber and a drawn PET fiber those can provide a cap ply cord and the like showing more improved modulus and good dimensional stability according to this, and a tire cord including the same.
(b) Description of the Related Art
A tire is a complex body of fiber/steel/rubber, and generally has a structure as illustrated in FIG. 1. That is, the steel and the fiber cords take a role of reinforcing the rubber and form a basic skeletal structure in the tire. It is, so to speak, like the role of a bone in a human body.
As a reinforcement of the tire, the performances such as fatigue resistance, shear strength, durability, repelling elasticity, adhesion to a rubber, and the like are required to the cord. Therefore, various cords made of suitable materials are used according to the performances required to the tire.
Recently, rayon, nylon, polyester, steel, aramid, and the like are generally used as the materials for the cord, and the rayon and the polyester are used for a body ply (or a carcass) (6 in FIG. 1), the nylon is mainly used for a cap ply (4 in FIG. 1), and the steel and the aramid are mainly used for a tire-belt part (5 in FIG. 1).
The structure and the characteristics of the tire represented in FIG. 1 are briefly described hereinafter.
Tread 1: A part contacting to the road surface; this part must provide a friction force necessary for braking and driving, be good in abrasion resistance, and also be able to stand against an external shock, and its heat generation must be small.
Body ply (or Carcass) 6: A cord layer inside the tire; this part must support a load and stand against a shock, and its fatigue resistance against bending and stretching movement during a driving must be good.
Belt 5: This part is located between the body plies and mostly composed of steel wire, and it lessens the external shock and also makes the ground contacting surface of the tread wide and the driving stability good.
Side wall 3: A rubber layer between the lower part of the shoulder 2 and the bead 9; it takes a role of protecting the internal body ply 6.
Bead 9: A square or hexagonal wire bundle, wherein a rubber is coated on the steel wires; it takes a role of fitting and fixing the tire to a rim.
Inner liner 7: A part located inside the tire instead of a tube; it makes a pneumatic tire possible by preventing air leakage.
Cap ply 4: A special cord fabric located on the belt of a radial tire for some passenger cars; it minimizes the movement of the belt during driving.
Apex 8: A triangular rubber packing material used for minimizing the dispersion of the bead, protecting the bead by relieving the external shock, and preventing an air inflow during shaping.
Recently, developments for tires suitable for high speed driving are required as the passenger cars gentrify, and accordingly the stability during high speed driving and high durability of the tire are recognized as greatly important characteristics. Furthermore, the performance of the materials for the cap ply cord importantly comes to the force before everything else for satisfying the characteristics.
The steel belt inside the tire is generally arranged in the oblique direction, the steel belt, however, tends to move toward the circumferential direction during high speed driving, and there are some problems that the sharp ends of the steel belt may cause separation between the layers of the belt and shape deformation of the tire by cutting the rubber or generating cracks. The cap ply prevents the separation between the layers and the deformation of the shape of the tire and takes a role of improving the high speed durability and the driving stability by restraining the movement of the steel belt.
A nylon 66 cord is mainly used for general cap ply cord. The nylon 66 cord can show the effect of restraining the movement of steel belt by generating high shrinkage force at the circumstance of high temperature corresponding to the internal circumstance of the tire during high speed driving, and wrapping the belt. However, it has disadvantages in that the nylon 66 cord may be deformed partially by the load of the tire and the car because it has low modulus and glass-transition temperature at high temperature and low dimensional stability according to this and it may clatter during driving due to the same.
To resolve the disadvantages, a PET cord having relatively high modulus and dimensional stability have been used as the cap ply cord, however, it is difficult to restrain the movement of the steel belt effectively because of its low shrinkage force, and it is also difficult to be applied to the cap ply cord. In addition, the shape of the PET cord may easily be deformed and the deformed PET cord results in a tire deformation when the load given to the cord is changed by the change of the driving speed of the car, because the cord composed of the general PET fiber does not have sufficient dimensional stability as well.
Furthermore, in case of the cord composed of high modulus low shrinkage (HMLS) PET fiber that is widely used as a fiber or an industrial fiber, it is possible to show high shrinkage force in comparison with the cord composed of the general PET fiber, however, the disadvantages like in the nylon 66 cord described above may still be occurred because the dimensional stability decreases as the modulus is decreased in this case.